I know you aren't a developer, but did you stop to think about what is going on behind the scenes of the login window?
There are literally a million and one things this could be, but you logic seems to be "It can't even handle a login screen!*"
That "one time" statement said it pretty clearly.
*Blissfully unaware of what goes on inside an operating system beyond the visible screen.
By the way, if you haven't formatted it yet, try an nvram reset. Shutdown the machine (hold down the power button until it turns off). Once you press the power button again hold down the following *ignore the parenthesis* (command,option, p, r). Hold them down until the machine restarts again and you hear the chime.
Hopefully that resolves it, but in general this is a good part of the troubleshooting procedures when you run into issues with a machine.
To say it frankly, this is why you don't use betas if you need that machine you're running on (if you don't have troubleshooting skills).
I hope the underlined helped.
Edit: Late to the party again I see :[
I don't have 10.10 but have you seen if safe mode is a working option?
Starting up in Safe Mode
To start up into Safe Mode (to Safe Boot), follow these steps.
Be sure your Mac is shut down.
Press the power button.
Immediately after you hear the startup tone, hold the Shift key.
The Shift key should be held as soon as possible after the startup tone, but not before the tone.
Release the Shift key when you see the gray Apple logo and the progress indicator (looks like a spinning gear).
After the logo appears, you should see a progress bar during startup. This indicates that your computer is performing a directory check as part of Safe Mode.
